1. Field of the Invention
The present invention relates to a magneto-optical recording/reproducing apparatus for recording or reproducing a digital information in or from an optical recording carrier, and particularly to a recording/reproducing apparatus for enlarging and reproducing a recording magnetic domain through domain wall displacement.
2. Related Background Art
There have been various recording media for respectively storing a digital information in a card or discoid medium. Among such recording media, a magneto-optical disk and its recording/reproducing apparatus are practically used in which a signal is written by raising the temperature of a medium by irradiation with a laser and simultaneously generating a magnetic field at the time of recording, and an information is reproduced in accordance with the polarized direction of the laser beam returned from the medium at the time of reproducing.
Recently, with these recording/reproducing apparatuses, there is a continuing need for improving the recording/reproducing density and new reproducing systems have been proposed in order to meet the need for density improvement. Particularly, the domain wall displacement detection is proposed in Japanese Patent Application Laid-Open No. 6-290496. The domain wall displacement detection is described below by referring to FIGS. 11A to 11C.
FIGS. 11A, 11B and 11C are schematic views for explaining a domain-wall-displacement magneto-optical recording medium and actions in its reproducing method.
FIG. 11A is a schematic sectional view of an example of a domain-wall-displacement magneto-optical recording medium. The magnetic layer of the medium is formed by sequentially stacking a first magnetic layer 11, a second magnetic layer 12 and a third magnetic layer 13. The arrow 14 in each layer denotes the direction of atomic spin. A domain wall 15 is formed at the boundary between regions whose spin directions are opposite to each other. Moreover, a recording signal SR of this recording layer is also shown as a graph at the lower side. The first magnetic layer 11 is formed by a vertical magnetic film having a relatively small domain-wall coercive force as compared to that of the third magnetic layer 13 at a temperature close to the ambient temperature and a large domain wall mobility, the second magnetic layer 12 is formed by a magnetic layer having a Curie temperature lower than those of the first magnetic layer 11 and the third magnetic layer 13 and the third magnetic layer 13 is formed by a vertical magnetic film.
FIG. 11B is a graph showing a temperature distribution (i.e., relation between position X and temperature T of medium) formed on the above magneto-optical recording medium. Though it is allowed that the temperature distribution is induced on the medium by a light beam applied for reproducing, it is preferable to form a temperature distribution by using another heating means together and raising a temperature from the front side of the spot of a reproducing light beam so that the peak of the temperature is positioned at the rear of the spot. In this case, at a position XS, the medium temperature is kept at a temperature TS close to the Curie temperature of the second magnetic layer 12. In the figure, TR represents room temperature.
FIG. 11C is a graph showing the distribution of a domain-wall energy density σ1, of the first magnetic layer 11 corresponding to the temperature distribution of FIG. 11B. In the figure, the left-hand ordinate indicates domain-wall energy density σ, the right-hand ordinate indicates forth F acting on domain wall, and the abscissa indicates position X. When a gradient of the domain-wall energy density σ1 is present in X-direction as shown in FIG. 11C, a force F1 shown by the following expression acts on the domain wall of each layer present at the position X.F1=∂σ1/∂x
The force F1 acts so as to displace a domain wall in the direction in which domain wall energy lowers. Because the first magnetic layer 11 has a small domain-wall coercive force and a large domain-wall mobility, a domain wall is easily displaced by the force F1. However, in the region before the position XS (right side in the drawing), the medium temperature is lower than Ts yet and the region is exchangeably coupled with the third magnetic layer 13. Therefore, the domain wall in the first magnetic layer 11 is also fixed to a position corresponding to the position of the domain wall in the third magnetic layer 13.
As shown in FIG. 11A, when the domain wall 15 is present at the position XS of the medium, the medium temperature rises up to the temperature Ts close to the Curie temperature of the second magnetic layer and the exchangeable coupling between the first and third magnetic layers is severed. As a result, the domain wall 15 in the first magnetic layer is “instantaneously” displaced to a region having a higher temperature and a smaller domain-wall energy density as shown by a broken line 17.
When the domain wall 15 passes the isothermal line of the temperature Ts formed below the spot 16 of the reproducing light beam, that is, in the vicinity of the front edge of the spot 16 in its traveling direction, all atomic spins of the first magnetic layer in the spot are unified in one direction. Moreover, whenever the domain wall 15 comes to the position XS in accordance with the movement of the medium, the domain wall 15 is instantaneously displaced below the spot, the direction of the atomic spin in the spot is inverted and all spins are unified in one direction. As a result, as shown in FIG. 11A, the reproduction signal amplitude becomes always constant and maximum independently of the interval between recorded domain walls (that is, recording-mark length) and is completely free from the problem of waveform interference or the like due to an optical diffraction limit. Therefore, it is possible to reproduce signals recorded at a high density independently of an optical diffraction limit.
Moreover, at the time of recording, the medium temperature is raised up to a Curie point with a light beam and a digital signal is recorded on the medium by an externally applied magnetic field during cooling. Specifically, recording is performed by formation of an edge (domain wall) of a recording mark at an isothermal line of the Curie temperature Tc of the third magnetic layer formed in the vicinity of the rear edge of the recording spot in its traveling direction.
However, an apparatus using the above-mentioned recording/reproducing principle has the following problem.
That is, because the edge of a recording mark is formed at the isothermal line of the Curie temperatureTc of the third magnetic layer in the vicinity of the rear edge of the light spot in its traveling direction at the time of recording, recording action is performed at a place shifted to the rear side when viewed from the central position of the light spot. FIG. 10 shows a relation between a recording signal (a) and the position of formation of a recording-mark (b) at the time of recording. In FIG. 10, in a case where the supply of the recording signal is started when the center of a light beam reaches a position (P), formation of the recording mark is started at a position X1 at the rear of the position (P) in the traveling direction.
In this case, when the recording start timing is the same as the reproducing start timing, because the displacement of a domain wall is started from a front edge of the light spot in its traveling direction at the time of reproducing, the reproducing of the recording mark is started from a position X2 shown by a broken line in (b) of FIG. 10. In this case, the recording mark formed between the positions X1 and X2 is not reproduced and therefore, omission of a reproduction signal is generated. This extremely deteriorates the reproduction signal quality or causes a reproducing unable state.
To solve the above problem, the reproducing start timing has been adjusted to be in agreement with the recording start timing by providing a given invalid period following the recording start timing, that is, shifting a recording-mark forming position in a data region. However, according to this method, the format efficiency is lowered because the invalid period is necessarily present at the head of the data region, so that the characteristic of the domain wall displacement detection, which is an advantageous high-density recording/reproducing system, cannot be completely exhibited.
Moreover, in the case of a format in which a sync pattern for realizing byte synchronization is present in a data region, a detection window signal for detecting a matching signal of the sync pattern is normally formed on the basis of the recording timing at the rearmost end of the sync pattern. However, when the recording timing is offset with respect to the reproducing timing as described above, the matching signal of the sync pattern is not housed within a detection window to cause a detection failure. To solve this problem, it is considered to increase the width of the detection window. However, a problem occurs that an incorrect sync-pattern-matching signal other than an original sync-pattern-matching signal may erroneously be detected.